HR:     09:35h
AN:     H11F-05 INVITED

TI:     Forest Roads, Chronic Turbidity, and Salmon

AU:     *Reid, L M
EM:     lreid/psw_rsl@fs.fed.us
AF:     Redwood Sciences Laboratory,
        USDA Forest Service,
        Pacific Southwest Research Station,
        1700 Bayview Drive
        Arcata, CA 95521  

AB:     Certain impacts of forest roads on habitats used by anadromous salmonids are widely 
        recognized and well-understood: road-related landslides increase sediment loads and modify 
        channel morphology, and culverts restrict access to parts of the channel network. Other 
        influences are less obvious, but may be even more pervasive. For example, road-related 
        erosion significantly increases chronic turbidity levels in streams. Flow and turbidity data from 
        Caspar Creek, California, were used to model the potential influence of the presence and use 
        of roads on cumulative duration curves for stream turbidity. Results suggest that a proportional 
        increase in fine-sediment production equivalent to that measured in coastal Washington (i.e. a 
        5.8-fold increase due to road-related erosion) would increase the average annual duration of 
        turbidities greater than 100 NTU by a factor of 73 (i.e. from 0.5 day to 36.5 days). Published 
        data suggest that feeding efficiency of juvenile coho salmon drops by 45\% at a turbidity of 100 
        NTU. Restriction of wet-weather road use would decrease the duration of higher turbidities by 
        about 30\% to 40\% on gravel-surfaced roads. Salmonid strategies for coping with high turbidity
        are likely to include use of off-channel, clean-water refugia and temporary holding at 
        clean-water tributary mouths. These coping strategies are partially defeated by the spatial 
        distribution of roads: road runoff discharges into low-order channels that once would have 
        provided clean inflows, and riparian roads restrict access to flood-plain and off-channel refugia.
        The temporal distribution of the high-turbidity inflows also decreases the effectiveness of 
        coping strategies: turbidities are high even during low-magnitude events when flows may not be
        sufficient to allow access to refugia. The combined influences of increased turbidity and 
        restricted opportunities for escape from the impact constitute a cumulative impact. Further, 
        traffic-related turbidity is highest during the day, when salmonids feed, and traffic produces 
        high turbidity even during small and moderate storm flows of autumn and spring, when water is 
        warmer than during winter floods. Because salmonid metabolic rates are 
        temperature-dependent, salmonids may be particularly sensitive to these unseasonal bouts of 
        high turbidity. 
        
DE:     1815  Erosion and sedimentation
DE:     1871  Surface water quality
DE:     1803  Anthropogenic effects
SC:     H
MN:     1998 Fall Meeting